Posts without metadata make nikola fail with cryptic error.

[SnoopJ]

When building a Nikola site with the ipynb compiler enabled, any notebooks not created or imported using nikola new_post will raise a ValueError from because of the missing metadata, e.g.:

ValueError: Invalid date '' in file posts/notes-on-meshgrid-ordering.ipynb

This is caused by the return of an empty dictionary in read_metadata().

I understand that this can be worked around with nikola new_post -i and creation of a separate metadata file, but the workflow I'd personally prefer when using Nikola is to author my notebooks in a way that is entirely Nikola-agnostic. Drawing inspiration from what's done for page metadata, I kludged together a solution in ipynb.py that works for my use case.

After speaking with @Kwpolska a little in Freenode #python, it appears this is partially a design decision, but I think it's pretty inelegant to just propagate the ValueError to the user.

For my workflow, my ideal user story would be:

I place an IPython notebook in posts/ and run nikola build
No Nikola metadata is found
The build script prompts me to enter the same info nikola new_post would ask for
After completing this process, the build script continues as normal

I'm happy to maintain my own version of the ipynb compiler for my personal use, but I figured I'd find out if this is a feature that would add value to the project. If not, then at the very least it seems appropriate to point the user to the new_post command when this error is raised.

[ralsina]

Build scripts are really not meant to be interactive. A better error message is indeed a good idea.

Also, this is not really ipynb specific, I am changing the title to reflect that.

[ralsina]

That branch mentioned above changes from that ... thing to this:

(nikola)ralsina@onalap1040 ~/P/n/foo> nikola build
Scanning posts.[2017-05-18T15:09:43Z] ERROR: Nikola: Missing date in file posts/foo.rst
[2017-05-18T15:09:43Z] ERROR: scan_posts: Error reading post posts/foo.rst
[2017-05-18T15:09:43Z] ERROR: Nikola: Error reading timeline
[2017-05-18T15:09:43Z] ERROR: Nikola: Error loading tasks